Driver circuits are common circuits used for transmission of signals, such as between integrated circuits (“ICs”) or within an IC. For example, driver circuits can be used to transmit (1) data to and from a memory or (2) control signals to and from a memory controller. A driver circuit receives and input signal, such as a data signal, and drives an output signal having a logic level based on the input signal. There are many types of drivers, each with their own advantages and disadvantages.
Typically, a driver circuit can generate a HIGH logic-level output signal, a LOW logic level output signal, or can be placed in a high-impedance (high-Z) state. Driver circuits having the three states (i.e., HIGH, LOW, high-impedance) are generally referred to as tri-state driver circuits, and placing a driver circuit into a high-Z state is often referred to as “tri-stating” the driver circuit.
One example of a conventional driver circuit is one where the driver circuit is tri-stated by using a separate enable signal. For example, while the driver circuit outputs a HIGH logic-level signal or a LOW logic-level signal, the enable signal is inactive so that voltage/current can be driven to the output of the driver circuit. However, when the enable signal is active, the driver circuit is placed into a tri-state condition, and although a logic level can be maintained at the output of the driver circuit, a high-impedance is presented at the output and a current is generally no longer driven.
With conventional driver circuits that use a separate enable signal to control the high-impedance state of the driver circuit, the timing of the enable signal relative to the input and output signals should be closely controlled so that the driver circuit enters and exits the tri-state condition without interfering with the driving of an output signal having the correct logic-level responsive the input signal. In ICs using relatively high clock speeds (e.g., GHz magnitude clock speeds), providing such a correctly timed enable signal often requires additional circuitry that consume area of a substrate on which the IC is fabricated and consumes electrical power.